Elastomeric films are used in products such as disposable diaper leg and waist bands and examination gloves. In the past, such elastomeric films have been made of elastomeric block copolymers, combinations of elastomeric block copolymers and liquid butadienes, combinations of elastomeric block copolymers and ethylene-vinyl acetate copolymers, thermoplastic urethanes, and combinations of copolyesters and ethylene-vinyl acetate copolymers. These materials have suitable properties for elastomeric films, such as low tensile set, low modulus, and high elongation, but each also has shortcomings. A primary shortcoming of many of these compositions is the inability to form a film by an extrusion-blowing process. Although other film forming processes, such as extrusion casting, solvent casting and calendering, are acceptable for many purposes, extrusion blowing is preferred for other applications. Extrusion will orientate polymer molecules in the extrusion direction and will cause tensile property loss in the direction transverse to the extrusion. Because blowing stretches the film in the direction transverse to the extruder, film produced by extrusion blowing has a superior directional balance of properties.
Elastomeric block copolymers, usually of vinyl arenes and conjugated dienes, may be used in elastomeric film. But these block copolymers which can be extrusion blown have melt processing temperatures which are too high for commercial extrusion blowers, and too high for commercially available antiblocking, antioxidant and stabilizer additives. The resultant extrusion blown film is therefore darkened and has a burned odor. Lower molecular weight block copolymers of vinyl arenes and conjugated dienes have melt temperatures which are acceptable for extrusion blowing, but may not be blown to an acceptably thin film. Tabana, in U.S. Pat. No. 4,017,436, discloses a composition of elastomeric block copolymer, liquid polybutadiene and tackifier which has film forming qualities superior to the elastomeric block copolymer alone. But the disclosed unhydrogenated block copolymer and unhydrogenated liquid polybutadiene would cause oxidative instability, thermal instability and intolerance to U.V. light which would result in unacceptably short shelf lives for many of the end uses of elastomeric films.
Elastomeric films containing thermoplastic urethanes, copolyesters and/or ethylene-vinyl acetate copolymers are expensive due to the cost of the raw materials. Films of these materials are also not as "soft" as would be desired for many applications. Items which are worn against the skin such as medical drainage apparatus, gloves and diapers would be preferably made from compositions which have a softer feel. The tensile set and modulus of films made of these materials are acceptable for many applications, but advantages would be realized from using a material with a lower tensile set and a lower modulus (a measurement of softness).
Compositions of a single vinyl arene-conjugated diene block copolymer, polystyrene and non-aromatic extender oil have been unsuccessfully tested by Applicants as extrusion blown elastomeric films. The block copolymers have such a high melt temperature that processing the compositions in commercial extruder-blowers with commercial additives, such as antiblocking agents and stabilizers, resulted in a discolored film with a smoky odor. Adding non-aromatic extender oil, adding polystyrene, and lowering the molecular weight of the block copolymer all could result in lower melt temperatures. Of these other methods of lowering melt temperature which were tested, however, none were found to result in a composition which could be extrusion blown.
It is therefore an object of this invention to provide a composition which is capable of extrusion blowing to a film of one to five mils in thickness, has high elongation, elastic recovery, and a low modulus. In another aspect, it is an object of this invention to provide an extrusion blown elastomeric film which has excellent elongation, has excellent elastic recovery, and has a low modulus. In an additional aspect, it is an object of this invention to provide a process for producing an elastomeric film which has an excellent elongation, has excellent elastic recovery, and has a low modulus.